Carbon materials which constitute negative electrodes of lithium ion secondary batteries have primarily been graphite materials and particularly artificial graphite powder.
There has been much research concerning how to increase the capacity per unit mass of graphite materials used for negative electrodes in order to increase the capacity of lithium ion batteries. As a result of such efforts, a graphite material has been developed which gives a capacity exceeding 360 mAh/g, which is close to the theoretical capacity of 372 mAh/g of graphite. Accordingly, increases in the capacity of graphite materials themselves have nearly reached a limit.
Under such circumstances, an attempt was recently made to increase the capacity of a negative electrode by more greatly compressing a negative electrode material and increasing the packing density of the negative electrode as a measure to increase the capacity of lithium ion batteries. In order to compress and densely pack a negative electrode material, it is necessary for particles of the negative electrode material to deform so as to fill empty spaces. Softer particles are more suitable for this purpose. If the particles are hard, it is necessary to apply an excessive load for compression, resulting in equipment problems.
In a lithium ion battery having a negative electrode constituted by a graphite material and particularly natural graphite powder which has a high crystallinity, decomposition of a nonaqueous electrolytic solution of the battery (and more particularly decomposition of an organic carbonate typically used as a solvent) readily occurs. This decomposition causes the irreversible capacity of the battery to increase, and the charge and discharge efficiency (the ratio of the discharge capacity to the charging capacity) and the cycling performance of the battery deteriorate. As a means of suppressing decomposition of an electrolytic solution, there have been many proposals of a carbon material in which a core made of a powder of a graphite material of high crystallinity has its surface coated with a carbonaceous material (see, for example, below-described Patent Documents 1-7). Inexpensive pitch is often used as a coating material. After the surface of the powder of a graphite material constituting cores is coated with pitch, the mixture of the graphite material powder and the pitch is subjected to heat treatment to carbonize the pitch, whereby the coating material becomes a carbonaceous material. In this manner, a carbon material having cores formed from a powder of a graphite material which are coated with a carbonaceous material is obtained.
The object of surface coating of a graphite material with a carbonaceous material which has been proposed in the past is to suppress a reaction between the graphite material and an electrolytic solution, thereby achieving an improvement in charge and discharge efficiency and cycling performance. Since reaction with an electrolytic solution takes place on the surface of the particles of the graphite material constituting cores, the entire surface of the core particles is coated with the carbonaceous material so that the surfaces of the cores are not exposed. A large amount of a coating material such as pitch is often used in order to coat the entire surface of the core particles. For example, in Patent Document 1, powder of a graphite material which constitutes cores and pitch are mixed in a mass ratio of the graphite material to pitch of 5/95-50/50, and then the mixture undergoes heat treatment. Accordingly, the proportion of pitch is 5-50 mass %.
Patent Documents 2-7 disclose a negative electrode material for a secondary battery having a multiphase structure which is formed from a core of a carbonaceous material having its surface coated with a surface layer of a carbonaceous material and which has a specific Raman spectrum and sometimes specific crystallographic or other properties. The microstructure of the carbonaceous material which forms the surface layer contributes to the Raman spectrum.
The methods disclosed in Patent Documents 2-7 for coating the surface of cores formed from a carbonaceous material are (1) coating by vapor phase thermal decomposition of an organic compound, (2) a method in which an organic compound in liquid phase is carbonized for coating (specifically, as employed in the examples, a fused polycyclic hydrocarbon material such as pitch is dissolved in an organic solvent to make liquid, and cores are immersed in the resulting solution (liquid phase) and then heat-treated to carbonize the pitch), and (3) a method in which cores are coated with an organic polymer such as a resin, and the coating material is subjected to thermal decomposition in solid phase. Any of these methods are intended to uniformly coat the entire surface of particles of the carbonaceous material constituting the cores.
The proportion of the surface layer in each of the carbon materials having a multiphase structure described in Patent Documents 2-7 is preferably 1-80 mass %, more preferably 5-70 mass %, and still more preferably 10-60 mass % in Patent Document 2; it is preferably 2-80 mass %, more preferably 5-65 mass still more preferably 5-50 mass %, and particularly preferably 6-40 mass % in Patent Documents 3 and 4; it is preferably 1-80 mass %, more preferably 5-60 mass %, and still more preferably 7-50 mass % in Patent Document 5; it is preferably 30-70 mass %, more preferably 35-75 mass %, and still more preferably 40-70 mass % in Patent Document 6; and it is preferably 10-65 mass %, more preferably 15-60 mass %, and still more preferably 20-55 mass % in Patent Document 7. However, the proportion constituted by the surface layer in the examples is 50 mass % in Patent Documents 2 and 6, 35 mass % in Patent Documents 3-5, and 45 mass % in Patent Document 7. There is no specific example of a multiphase structure in which the proportion constituted by the surface layer is smaller than 35 mass %. The reason why the proportion constituted by the surface layer is given a large value in this manner is thought to be in order to impart a desired Raman structure by the microstructure of the surface layer.    Patent Document 1: JP 2003-100292 A1    Patent Document 2: JP H10-321218 A1    Patent Document 3: JP H10-255851 A1    Patent Document 4: JP H05-94838 A1    Patent Document 5: JP H05-217604 A1    Patent Document 6: JP H05-307976 A1    Patent Document 7: JP H05-307977 A1